Published on Web 02/06/2007
Electrochemical Generation of Cationic Pd Catalysts and Application to Pd/TEMPO Double-Mediatory Electrooxidative Wacker-Type Reactions Koichi Mitsudo,* Takashi Kaide, Eriko Nakamoto, Kenta Yoshida, and Hideo Tanaka* DiVision of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama UniVersity, 3-1-1 Tsushima-Naka, Okayama 700-8530, Japan Received December 29, 2006; E-mail:
[email protected];
[email protected] Cationic palladium complexes with weakly coordinating anions have received much attention as catalysts. Over the past decade, many kinds of cationic palladium-catalyzed reactions have been reported.1 The most common methods for preparing cationic palladium PdLmRn (L ) ligands; R ) anion, e.g., BF4-, PF6-, and ClO4-) are (i) the reaction of a palladium halide complex with metalated anion counter MR (M ) Na+, Ag+) and (ii) the reaction of Pd sponge and nitrosonium salt (NOR).
Table 1. Electrooxidative Synthesis of Cationic Palladium Catalysts
entry
electrolyte
product
yield (%)a
1 2 3
Bu4NBF4 Bu4NPF6 Bu4NClO4
[Pd(CH3CN)4][BF4]2 (1a) [Pd(CH3CN)4][PF6]2 (1b) [Pd(CH3CN)4][ClO4]2 (1c)
58 82 (58)b 64
nMR
(i) PdLmXn 9 8 PdLmRn -nMX mL, nNOR
(ii) Pd 9 8 PdLmRn -nNO We have been interested in electrooxidative molecular transformations2 and, during the course of our study, we extended our research to the oxidative formation of metal complexes. In our preliminary study, we found that palladium acetate was readily electrooxidized at the anode to give cationic palladium species. We sought to improve the electrooxidation reaction to develop a method for the synthesis of cationic palladium and to use the resulting cationic palladium as a catalyst. The potential advantages of the electrooxidative generation system of cationic palladium catalysts are that (i) air and moisture sensitive active catalysts can be generated in situ from stable precursors and (ii) the system can be easily integrated into electrooxidative reactions. We report here the electrooxidative synthesis of cationic palladium complexes [Pd(CH3CN)4][R]2 (R ) BF4-, PF6-, and ClO4-) and their application to Wacker-type electrooxidation.3 First, we attempted the electrooxidation of Pd(OAc)2 in an undivided cell, but palladium metal was deposited on the cathode and the desired metal complex was not obtained. To avoid the reduction of Pd2+, we next conducted the electrooxidation in a divided cell (Table 1). Passage of 3 F/mol of electricity through a CH3CN solution of Pd(OAc)2 in the presence of tetrabutylammonium tetrafluoroborate (Bu4NBF4) gave [Pd(CH3CN)4][BF4]2 (1a) in 58% yield (entry 1).4 In this system, the counteranion would come from the electrolyte. With Bu4NPF6, [Pd(CH3CN)4][PF6]2 (1b) was formed in 82% yield (entry 2). Bu4NClO4 could also be used in the system as an electrolyte to give [Pd(CH3CN)4][ClO4]2 1c (64%, entry 3). It is likely that the reaction proceeded via a Kolbe-type reaction.5 A plausible mechanism is illustrated in Scheme 1. First, the acetate anion of Pd(OAc)2 would be oxidized on the anode to Pd2+ and two molar amounts of acetoxy radical (•OAc). Subsequent decarboxylation would generate carbon dioxide and methyl radical, which would immediately dimerize to give ethane. The remaining cationic Pd2+ species, probably stabilized by the coordination of CH3CN, might react with the anion part of the electrolyte to generate the corresponding cationic palladium species. 2246
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J. AM. CHEM. SOC. 2007, 129, 2246-2247
a
Isolated yield.
b
A total of 2 F/mol of electricity was passed.
Scheme 1. Plausible Mechanism for the Generation of Cationic Palladium Complex 1
Scheme 2. N-Oxyl/Pd Double-Mediatory Electrooxidation of 2a
To demonstrate the practical utility of this system, the electrooxidative generation system was integrated into an electrooxidative Wacker-type reaction (Scheme 2). Passage of 3 F/mol of electricity through a CH3CN/H2O (7/1) solution of 1-dodecene 2a (0.5 mmol), Pd(OAc)2 (10 mol %), and TEMPO (30 mol %) in the presence of Et4NOTs (0.05 M) as a supporting electrolyte formed 3a in 59% yield. The addition of TEMPO was essential for the reaction: without TEMPO, only a trace amount of 3a was obtained. Noteworthy is that other isomerized ketones were not obtained in the reactions.6 Using Pd(TFA)2 instead of Pd(OAc)2, the yield of 3a decreased to 20%. The presence of a phosphine ligand drastically suppressed the reaction. With PdCl2(PPh)2, only a trace amount of the oxidized product 3a was obtained, and most of 2a was recovered. Among the palladium sources that have been examined so far, Pd(OAc)2 gave the best result, suggesting that Kolbe-type decomposition of acetate anion promoted efficient formation of the active palladium species.7 We assumed that TEMPO might be required for the effective oxidation of Pd(0) to Pd2+.8 We next optimized the electrolyte for the electrochemical Wacker-type oxidation (Table 2). As expected, the efficiency of the Wacker-type reaction was highly affected by the electrolyte. With Et4NBF4, Et4NPF6, or Et4NClO4, ketone 3a was obtained in respective yields of 69%, 66%, and 77%. With Et4NNTf2, the yield of the ketone decreased to 48% (entry 4). With Et4NOAc, the 10.1021/ja069043r CCC: $37.00 © 2007 American Chemical Society
COMMUNICATIONS Table 2. N-Oxyl/Pd Double-Mediatory Electrooxidation of 2a Using Various Supporting Electrolytes
a
entry
electrolyte
conversion (%)
yield (%)a
1 2 3 4 5 6 7 8
Et4NBF4 Et4NPF6 Et4NClO4 Et4NNTf2 Et4NOAc LiClO4 NaClO4 KClO4
92 95 100 85 19 100 100 100
69 66 77 48 4 83 82 80
the reaction using 1-tetradecene (2d) gave tetradecan-2-one (3d) in 91% yield (entry 3). We next examined the tolerance of functional groups under these conditions, and found that a variety of functional groups could endure the reaction conditions. For instance, the reaction of substrates bearing ester or phenyl groups gave the corresponding ketones in moderate to good yields (entries 4 and 5). Notably, a formyl group was not oxidized under these conditions. The oxidation of undec-10-enal (2g) formed 10-oxoundecanal (3g) in 75% yield (entry 6). In summary, we have developed an electrochemical method for generating cationic palladium complexes [Pd(CH3CN)4][X]2, and then used them for the in situ generation of the reagent for accomplishing the electrooxidative Wacker-type reaction. Further investigation of this strategy is in progress in our laboratory. Acknowledgment. We thank the SC-NMR Laboratory of Okayama University for 1H and 13C NMR analyses.
Isolated yield.
Table 3. N-Oxyl/Pd Double-Mediatory Electrooxidation of Various Olefins
Supporting Information Available: Experimental procedures and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. References
a
Isolated yield.
Wacker-type reaction did not proceed smoothly, and 81% of starting material 2a was recovered (entry 5). On the basis of these results, ClO4- was determined to be the most effective anion moiety for the reaction. Next, we investigated the effect of the cationic part of the electrolyte. The cationic part did not have as great influence as the anionic moiety. With LiClO4 or NaClO4 as the electrolyte, the yield of 3a increased slightly to 83% or 82%, respectively (entries 6 and 7). Finally, we carried out the Wacker-type oxidation using other olefins in a similar electrooxidation system (Table 3). Higher molecular weight terminal alkenes, which are hardly oxidizable substrates for the Wacker reaction because of their low solubility in polar solvents, could be used, and the corresponding ketones were obtained in moderate to high yields (entries 1-3). Especially,
(1) For reviews of recent works, see: (a) Jutand, A. Appl. Organomet. Chem. 2004, 18, 574. (b) Jutand, A. Eur. J. Inorg. Chem. 2003, 2017. (c) Widenhoefer, R. A. Acc. Chem. Res. 2002, 35, 905. (d) Hamashima, Y.; Sodeoka, M. Chem. Rec. 2004, 4, 231. (e) Strukul, G. Top. Catal. 2002, 19, 33. (f) Belov, G. P. Russ. Chem. Bull., Int. Ed. 2002, 51, 1605. (g) Slagt, M. Q.; van Zwieten, D. A. P.; Moerkerk, A. J. C. M.; Klein Gebbink, R. J. M.; van Koten, G. Coord. Chem. ReV. 2004, 248, 2275. (h) Mecking, S. Coord. Chem. ReV. 2000, 203, 325. (i) Pascu, S. I.; Coleman, K. S.; Cowley, A. R.; Green, M. L. H.; Rees, N. H. J. Organomet. Chem. 2005, 690, 1645. (j) Drent, E.; Budzelaar, P. H. M. Chem. ReV. 1996, 96, 663. (2) (a) Suematsu, S.; Mitsudo, K.; Katagiri, F.; Tanaka, H. Electrochemistry 2007, 75, 54. (b) Tanaka, H.; Arai, S.; Ishitobi, Y.; Kuroboshi, M.; Torii, S. Electrochemistry 2006, 74, 656. (c) Mitsudo, K.; Matsuda, W.; Miyahara, S.; Tanaka, H. Tetrahedron Lett. 2006, 47, 5147. (d) Kubota, J.; Ido, T.; Kuroboshi, M.; Tanaka, H.; Uchida, T.; Shimamura, K. Tetrahedron 2006, 62, 4769. (e) Kubota, J.; Shimizu, Y.; Mitsudo, K.; Tanaka, H. Tetrahedron Lett. 2005, 46, 8975. (f) Mitsudo, K.; Kawaguchi, T.; Miayahara, S.; Matsuda, W.; Kuroboshi, M.; Tanaka, H. Org. Lett. 2005, 7, 4649. (g) Tanaka, H.; Kubota, J.; Miyahara, S.; Kuroboshi, M. Bull. Chem. Soc. Jpn. 2005, 78, 1677. (h) Kuroboshi, M.; Fujisawa, J.; Tanaka, H. Electrochemistry 2004, 72, 846. (i) Tanaka, H.; Chou, J.; Mine, M.; Kuroboshi, M. Bull. Chem. Soc. Jpn. 2004, 77, 1745. (j) Tanaka, H.; Kubota, J.; Itogawa, S.; Ido, T.; Kuroboshi, M.; Shimamura, K.; Uchida, T. Synlett 2003, 951. (k) Tanaka, H.; Kawakami, Y.; Goto, K.; Kuroboshi, M. Tetrahedron Lett. 2001, 42, 445. (l) Kuroboshi, M.; Yoshihisa, H.; Cortona, M. N.; Kawakami, Y.; Gao, Z.; Tanaka, H. Tetrahedron Lett. 2000, 41, 8131. (3) (a) Tsuji, J.; Minato, M. Tetrahedron Lett. 1987, 28, 3683. (b) Inokuchi, T.; Ping, L.; Hamaue, F.; Izawa, M.; Torii, S. Chem. Lett. 1994, 441. (4) Without electrolysis, the cationic complex was not obtained, and Pd(OAc)2 was recovered. (5) For reviews concerning the Kolbe reaction, see: (a) Torii, S.; Tanaka, H. In Organic Electrochemistry, 4th ed.; Lund, H., Hammerich, O., Eds.; Marcel Dekker, Inc.: New York, 2001; 499-543. (b) Moeller, K. D. Tetrahedron 2000, 56, 9527. (c) Utley, J. Chem. Soc. ReV. 1997, 26, 157. (6) Torii and co-workers reported that the isomerized ketones were obtained as by-products from higher molecular weight alkenes in the electrooxidative Wacker-type reactions using (p-Br-C6H4)3N or hydroquinone as a mediator.3b (7) Hayashi and Uozumi reported the high efficiency of cationic Pd, catalysts in a Wacker-type reaction, see: Uozumi, Y.; Kato, K.; Hayashi, T. J. Org. Chem. 1998, 63, 5071. (8) In the presence of triphenylamine and Bu4NX, the electrooxidation of Pd(OAc)2 gave cationic Pd complex [Pd(CH3CN)4][X]2. On the other hand, when Pd(OAc)2 was electrooxidized in the presence of TEMPO, the Pd complex bearing TEMPO was also obtained. The complex also might work as an active catalyst for the Wacker-type reaction. While the exact structure has not been clear yet, elemental analyses suggested that the complex might be [Pd(CH3CN)4(tempo)][X]. The identification of the exact structure is undertaken.
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